Conventionally, known metal halide lamps as a light source for use in stores and the like have a triple-tube structure in which an arc tube, an inner tube surrounding the arc tube, and an outer tube surrounding the inner tube are provided such that the longitudinal central axes of the respective tubes are substantially coincident with each other (see, for example, Patent document 1). The arc tube is provided with a pair of electrodes therein, and is filled with a metal halide (light emitting metal), mercury, and inert gas.
The inner tube has a tip-off portion as a remaining part of an exhaust pipe in one end portion thereof, and has a sealing portion formed of a collapsed open end portion in the other end portion. Further, a space inside the inner tube is maintained under vacuum or is filled with nitrogen gas.
The inner tube frequently is made of quartz glass with an ultraviolet protection property to which cerium (Ce) or titanium (Ti), for example, is added to block ultraviolet rays emitted from the arc tube.
One end portion of the outer tube is closed in a substantially hemispherical shape, and a stem is adhered to the inside of the outer tube in the other end portion. Further, a base is attached to the outside of the outer tube in the other end portion. Stem lines are adhered to the inside of the stem. One end portion of the stem lines is connected electrically to the base, and the other end portion thereof is introduced into the outer tube to hold the inner tube and supply power to the electrodes.
The outer tube frequently is made of high-shock-resistant hard glass so that it is not damaged easily even if shattered pieces of the arc tube collide with the outer tube or an external shock is applied to the outer tube during transport.
The metal halide lamp having the triple-tube structure ensures excellent safety since the outer tube is not damaged easily even if the arc tube is destroyed. Therefore, this metal halide lamp is suitable for use in combination with a bottom-surface-open-type lighting unit equipped with no front glass or the like.
A bottom-surface-open-type lighting unit is used as a lighting unit for spotlight. A lighting unit for spotlight for use in stores and the like is required to be remarkably compact in size. For this reason, a halogen lamp, which is more compact than a metal halide lamp, has been used as a light source to be incorporated into a lighting unit for spotlight for use in stores and the like.
However, metal halide lamps are more efficient and have a longer life than halogen lamps. Thus, it has been demanded to use a metal halide lamp instead of a halogen lamp as a light source to be incorporated into a bottom-surface-open-type lighting unit for spotlight. Among metal halide lamps, a ceramic metal halide lamp in which an arc tube includes an envelope composed of translucent ceramic is expected as an alternative to a halogen lamp. For example, in the case of a ceramic metal halide lamp that consumes 20 W or 35 W of power, an arc tube is remarkably compact (for example, having a maximum outer diameter of 4 mm to 6 mm and an entire length of 25 mm to 35 mm), and yet it is possible to deliver luminance equal to that of a halogen lamp with about ⅓ the power consumed by the halogen lamp.
Patent document 1: JP 8 (1996)-236087 A